1. Field of the Invention
This invention relates to a diene derivative useful for liquid crystal materials. More particularly, it relates to a liquid crystalline compound containing a conjugated diene inside its molecule and a liquid crystal composition containing the same.
2. Description of the Related Art
Display elements making use of liquid crystals have been broadly utilized for clocks, electronic computers, etc. These liquid crystal display elements utilize the optical anisotropy and dielectric anisotropy of liquid crystal substances. Liquid crystal phases include a nematic liquid crystal phase, smectic liquid crystal phase and cholesteric liquid crystal phase. Among these phases, display elements utilizing nematic liquid crystal have been most broadly practically used. Further, display modes applied to the liquid crystal display includes TN (twisted nematic) mode, DS (dynamic scattering) mode, guest-host mode, DAP mode, etc. Many liquid crystalline compounds including those at a studying stage have been known, but at present, there is no single substance which is sealed in a display element and used. This is because while liquid crystal substances expected to be used as display elements are preferred to be those which exhibit liquid crystal phase within a temperature range as possible as broad around a room temperature at which they are most often used as display elements, in the natural fields, and further they should be sufficiently stable to environmental factors used and have sufficient physical properties for driving the display element, no single substance satisfying these conditions has not yet been found. Thus, at present, compositions having such characteristics have been prepared by mixing several kinds of liquid crystal materials or further mixing non-liquid crystalline compounds therewith, and have been practically used as liquid crystalline materials. Further, these liquid crystal compositions are required to be stable to moisture, light, heat, air, etc. usually present under the environment where they are used. Further, the compositions are required to be stable to electric field and electromagnetic irradiation, and further the liquid crystalline compounds to be mixed are required to be chemically stable to one another under the environment where they are used. Further, the values of various physical properties such as optical anisotropy value, dielectric anisotropy value, conductance value, etc. are required to be suitable values corresponding to a display mode and a shape of element. In particular, an importance of a substance having a low optical anisotropy value as a material for liquid crystal display element of thin film transistor (TFT) mode and a compound having a large elastic constant ratio (K.sub.33 /K.sub.11) as a material for super twisted nematic (STN) mode has been recently increasing. In order to correspond to such requirements, single substance liquid crystalline compounds having various characteristics have been developed, and as a compound for improving particularly the values of the viscosity and elastic constant ratio, compounds having a carbon chain having an unsaturated bond at the terminal part of the molecule, as mentioned below, has been found (Japanese patent application laid-open No. Sho 59-176221). Further, as a compound having a rigid molecular structure, a compound having a dienyl group, as mentioned below, has been developed (Japanese patent publication No. Hei 2-207056). However, the compound disclosed in the latter publication has been limited to that having the core part which is most important for exhibiting liquid crystallinity ether-bonded to the dienyl group. Further, as to the core part, too, the compound has been limited to that having 4-(4-phenyl)ethynylphenyl (tolan) structure. Thus, the compound has a high viscosity and is poor in utility, as a liquid crystalline material.
Thus, it has been required to develop a new material which can solve these problems.
A compound disclosed in Japanese patent application laid-open No. Sho 59-176221: ##STR2##
A compound disclosed in Japanese patent publication No. Hei 2-207056: ##STR3## The object of the present invention is to provide a liquid crystalline compound having improved values of the viscosity and the elastic constant ratio.
The present inventors have made extensive research in order to solve the above-mentioned problems, and as a result, have found a compound having a novel structure and having improved characteristics as compared with generally known liquid crystalline compounds, and have completed the present invention.
The present invention is characterized by a diene derivative expressed by the formula (I) ##STR4## wherein R and R' each independently represent a hydrogen atom or an alkyl group of 1 to 5 carbon atoms; A and B each independently represent 1,4-cyclohexylene or 1,4-phenylene; n represents 0 or 1; m represents an integer of 0 to 2; X and X' each independently represent a hydrogen atom or a fluorine atom when A represents 1,4-phenylene, and a hydrogen atom when A represents 1,4-cyclohexylene; and Y represents an alkyl group, a halogen atom, a cyano group, an alkoxy group, a methyl group substituted by 1 to 3 halogen atoms or a trihaloalkoxy group when A represents 1,4-phenylene, or an alkyl group, an alkoxy group; a methyl group substituted by 1 to 3 halogen atoms or a trihaloalkoxy group when A represents 1,4-cyclohexylene group; and R and R' do not represent simultaneously an alkyl group of 4 or more carbon atoms.